Jiung Jang1,Hyo Jae Yoon1
Korea University1
Jiung Jang1,Hyo Jae Yoon1
Korea University1
In molecular thermoelectrics, study of thermopower in molecular junction permits access to physical-organic information about nano-scale thermoelectric energy conversion. Whereas π-extended building blocks have been considered as efficient thermoelectric materials, aliphatic molecules are usually avoided, and the related mechanism of thermoelectric energy conversion remains unclear. In this presentation, we show that replacement of methyl terminal group with carboxylic acid significantly enhances thermopower in self-assembled monolayer (SAM)-based large-area molecular junction. Using the liquid metal junction technique, we formed non-damaging contacts over the delicate surface of SAMs and examined length dependence of Seebeck coefficient. We focused on the following three different series of aliphatic molecules and compared their Seebeck coefficient: mercaptoalkanes (HS(CH<sub>2</sub>)<sub>n-1</sub>CH<sub>3</sub>; n=2-10, 12, 14, 16, 18), mercaptoalkanoic acids (HS(CH<sub>2</sub>)<sub>n-1</sub>CO<sub>2</sub>H; n=2-8, 11, 12, 16), and methyl mercaptoalkanoates (HS(CH<sub>2</sub>)<sub>n-1</sub>CO<sub>2</sub>CH<sub>3</sub>; n=2-8, 11, 12). Two interesting features were observed. i) The alkanoic acid molecules led to significant increases in the Seebeck coefficient by up to ~250 %, compared to the alkane molecules of similar lengths. ii) Rather surprisingly, in the short molecular-length regime (n≤5), Seebeck coefficient of alkanoic acid molecules increased as the length of aliphatic backbone increased, whereas the others (mercaptoalkane and mercaptoalkanoate molecules) exhibited linear regression of Seebeck coefficient with increasing the length. Structural analysis over the SAMs and theoretical calculations suggested that the unusual thermoelectric behaviors were attributed to the interplay between the supramolecular disorder in SAM and the electronic interaction between the carboxylic acid terminal group and top electrode, which had significant influence on the energy offset between Fermi level and energy level of accessible occupied molecular orbital.